Log periodic antennas have several desirable properties including maintaining a relatively constant radiation pattern over relatively large frequency bands, that is, they are considered broadband antennas. In the construction of most log periodic antennas the radiating element is a tapered or triangular plane comprising conductor elements extending from the apex or vertex of the triangle using either wire elements or conducting patterns on an insulating substrate. Multiple element log periodic antennas usually have four or more elements, where each element is generally triangular in shape and has a vertex and sides defined by an angle .alpha.. More specifically, each element is comprised of at least two radial sections defined on a common side of a center line of the antenna element and on the other side by a radial line extending from the vertex at an angle .alpha./2. Various configurations of the radial sections are employed for the log periodic antenna.
With the log periodic antenna, it is frequently desirable to use a single antenna structure which has at least two distinct and separate modes of operation, each being excited by its own source by means of a separate coaxial feedline. An advantage of the log periodic antenna is that it can be made to operate in the two separate modes without expensive and frequency limiting connecting networks.
It is also frequently desirable to operate a log periodic antenna with one or more types of energy polarization, that is, linear polarization, circular polarization, or elliptical polarization. Heretofore, one antenna capable of operating in a variety of polarization types is known to those skilled in the art as a "four-arm cavity backed multi-mode spiral". Such an antenna consists of a four-arm spiral element installed in front of a cavity with the radio frequency energy from the four-arms of the spiral combined in a special manner by a network so as to allow two or more simultaneous modes of operation. Each mode has its own special and unique radiation pattern characteristics. While the four-arm spiral element antenna is log periodic in nature and therefore broadband, the cavity is not broadband and the combining network has certain frequency limitations. Therefore, although the "four-arm cavity backed multi-mode spiral" antenna is capable of being operated in a variety of polarization directions it has only a limited band width capability.
Primarily, the four-arm cavity backed multi-mode spiral antenna is used, when receiving radio frequency energy, to determine the direction of arrival of the received energy. Measurements are made of amplitude and phase of the received radio frequency energy at each of the coaxial feedlines. By use of an appropriate algorithm in a computer network these measurements can be used to determine the angle of arrival at the radio frequency energy. Such an antenna has a characteristic known as "axis rotation with frequency" and either the frequency of the received energy must be known or a special compensating network must be incorporated into the combining network to fully determine the direction of arrival.
Log periodic monopole antennas have also been used in conjunction with flat conducting planes for transmitting and receiving of radio frequency energy. The use of a flat conducting plane, however, does not permit the arraying of the log periodic monopole antenna elements thereby limiting such structures to only radio frequency energy polarized parallel to the antenna elements for both the transmit and receive modes.
Construction details for both the monopole elements and supporting structure for a log periodic antenna varies considerably as evidenced by the number of U.S. patents issued covering such antennas. Typically, a prior art antenna structure employing log periodic elements to transmit and receive elliptically polarized radio frequency energy consists of four log periodic elements arranged in quadrature pairs. The elements, as explained, have a generally triangular shape with the vertices of the four elements located close together. Basically, the overall antenna has a pyramidal shape outline with each of the four antenna elements forming one face of the pyramid. The edges of the adjacent elements do not make contact with each other being spaced apart a small distance. Of course, this is only an overall description of the construction of log periodic antennas, but all such antennas basically have the same general outline.